The invention relates to a device and a method for measuring blood pressure.
The device and the method serve to measure blood pressure, namely to measure blood pressure in a non-invasive manner. The term "non-invasive" here means that the measurement is performed without an instrument being introduced into a blood vessel and is thus effected with sensor means which are disposed completely outside of the living human, or possibly animal body whose blood pressure is being measured.
At present, blood pressure is mostly measured by methods based on the Riva-Rocci method. Prior art devices provided for such blood pressure measurements include a deformable cuff. This cuff defines a cavity which is connected with a compressed gas source, usually formed by a pump that pumps air, an outlet and a pressure measuring device. Means are further provided to be able to associate two values of this pressure when there is a change in the pressure existing in the cuff--namely upon deflation of the cuff--with the systolic and the diastolic pressure. The association with the systolic and the diastolic pressure can here be made either on the basis of Korotkoff sounds generated when the blood flows through an artery or according to the oscillometric variant of the method. In more recent prior art sphygmomanometers, the pressure measuring devices include a measuring transducer that is connected with the cavity in the cuff for converting the pressure into an electrical value, electronic circuit means and a display member for the analog or digital display of the systolic and diastolic blood pressure. Devices for determining the systolic and diastolic blood pressure on the basis of the Korotkoff sounds additionally include either a stethoscope or a microphone. Reference is here made, for example, to German Laid-Open Patent Application 3,014,199 and the corresponding U.S. Pat. No. 4,459,991. The pulsating flow of the blood is able to excite vibrations in the gas present in the cuff, normally air. In the devices provided for oscillometric measurements, the pressure measuring transducer and the electronic circuit means are configured to detect the fluctuations of the pressure in the cuff connected with the above-mentioned vibrations.
For a measurement according to the Riva-Rocci method, the cuff is fastened to a body segment--for example an upper arm or a finger--and is pumped up until the pressure of the air present in its cavity is sufficient to constrict the artery in the enclosed member. Then the cuff is slowly deflated. In the variant involving the detection of the Korotkoff sounds by means of a stethoscope or microphone, two values are detected and identified for the pressure in the cuff cavity during deflation of the cuff as the systolic blood pressure and the diastolic blood pressure, respectively. The pressure existing in the cuff during the first occurrence of Korotkoff sounds is associated with the systolic blood pressure. The diastolic pressure is recognized by the fact that the actual Korotkoff sounds disappear, with the sounds generated by the flowing blood becoming lower and less distinct or disappearing altogether. In the oscillometric variant of the method, the pressures of the air contained in the cuff and corresponding to the systolic and diastolic blood pressures are determined in that the fluctuations in the cuff pressure caused by the pulsating flow of the blood begin to appear or disappear again.
For seriously ill or critical accident victims and/or patients just coming out of surgery and in other cases it may be necessary or at least desirable to measure the blood pressure of the respective patient over a certain period of time--for example over several hours or days--permanently and as continuously as possible. In practice, devices are known for this purpose which operate according to the Riva-Rocci method and in which the cuff can be inflated and deflated automatically in cycles during operation, with the systolic and diastolic blood pressure each being measured during the deflation. However, periodic pumping up and subsequent deflating of the cuff and the interruption of blood circulation connected therewith in the limb around which the cuff is placed is unpleasant for the patient being examined and may even be damaging to his health. Since an inflation/deflation cycle usually requires at least about one minute and, moreover, short pauses should be introduced between successive measurements to keep annoyance to the patient being examined at a minimum, the Riva-Rocci method does not really permit truly continuous blood pressure measurements.
The publication entitled "Possible Determinants of Pulse-Wave Velocity In Vivo" by Masahiko Okada, in IEEE Transactions on Biomedical Engineering, Volume 35, No. 5, May 1988, pages 357-361, discloses a photoplethysmographic method for measuring pulse wave velocity that will be discussed in greater detail below. The measurement is made at the finger or toe tips with the use of light at a wavelength of 300 nm to 500 nm. This publication describes the correlation of the pulse wave velocity with various other parameters and variables, one of which is the blood pressure. According to this publication, a certain correlation was found to exist between the pulse wave velocity and the systolic and diastolic blood pressure. Such a relatively slight correlation, however, does not permit a determination of the blood pressure. Since the pulse wave velocity does not change periodically, it would also not be possible, in particular, to determine the systolic and the diastolic blood pressure from the pulse wave velocity. Moreover, the walls of the large arteries and the tissue portions usually covering them toward the exterior are practically impermeable to light of a wavelength of 300 nm to 500 nm. The method disclosed in the publication by M. Okada is therefore suitable only for measurements at thin-walled blood vessels near the surface, which are correspondingly small and is not suitable for measurements at large, correspondingly thick-walled blood vessels that may possibly be relatively far removed from the surface of the body part being examined.
Several general characteristics relating to blood circulation will now be discussed. The circulatory system includes arterial blood vessels -(that is, arteries),-venous blood vessels, and capillaries that interconnect the two types of vessels. The smallest arterial blood vessels or arteries, that are connected directly with the capillaries, are called arterioles. The arterial blood vessels have elastically deformable walls and are at least in part provided with muscle fibers and/or enclosed by such muscle fibers. These muscle fibers are able to compress the arteries and particularly the arterioles to different degrees and thus influence their elasticity, the flow resistance and the distribution of blood to the various blood vessels. The heart pumps the blood in a pulsating manner--that is, in surges--through the blood vessels. The blood flows through the blood vessels at a flow velocity v that is a function of locus as well as time. If, for the sake of simplification, it is initially assumed that the blood vessels have rigid walls, changes in pressure in the blood propagate at the speed of sound cs, whose second power or square is defined by the following formula: EQU c.sub.s.sup.2 =k/.rho. (1)
where .rho. is the density of the blood and K the modulus of compression, which is also called the volume elasticity modulus and is equal to the reciprocal of compressibility, usually identified as .kappa..
In reality, however, the arterial blood vessels do not have rigid walls but--as already mentioned--elastically deformable walls. During each blood surge caused by one cardiac cycle and the pulse-like pressure increase connected therewith, the arterial blood vessels are distended. These distensions propagate along the arterial blood vessels. The velocity at which the change in pressure caused by a cardiac cycle or blood surge propagates along an arterial blood vessel under the influence of its wall elasticity, is the already mentioned pulse wave velocity c.sub.pw. According to the book by Ludwig Prandtl, entitled "Furer durch die Stromungs-lehre" [Fluid Mechanics Guide], published by Verlag Friedr. Vieweg & Sohn, Braunschweig, 1965, the second power or square of the propagation velocity of pressure changes in tubes having elastically distensible walls, and thus at least approximately also the second power or square of the pulse wave velocity, neglecting flexural vibrations, is given by the following equation: ##EQU1## where E is the modulus of elasticity of the blood vessel wall, s is the thickness of the blood vessel wall and d is the interior diameter of the blood vessel.
According to the above-cited publication by M. Okada, the square of the pulse wave velocity is given by the following equation: ##EQU2##
By inserting c.sub.s in Equation (2), it can be demonstrated that Equation (3) is derived from Equation (2) if, for the sake of simplicity, the second product in the parenthetical expression in Equation (2) is omitted.
The flow velocity of the blood is--as already mentioned--a function of locus as well as time. Its maximum value in an arterial blood vessel and particularly in a large artery of a grown human being is at most about 0.5 m/s and normally a little less. According to Equations (2) and (3), the pulse wave velocity is dependent upon the ratio of the wall thickness to the diameter of the arteries. Since this ratio increases from the heart toward the capillaries and since the pulse wave velocity additionally is a function of the modulus of elasticity and of the tension in the muscle fibers belonging to the respective blood vessel, the pulse wave velocity changes along the arterial blood vessels and is also dependent upon the state of the human beings or animals examined. In the arteries, the pulse wave velocity is typically about 4 m/s to 5 m/s. The speed of sound in water, which is known to be the major component of blood, lies in an order of magnitude of 1500 m/s. The pulse wave velocity c.sub.pw is thus significantly greater, namely at least or approximately 10 times greater, than the flow velocity v, and the speed of sound c.sub.s, in turn, is very much greater than the pulse wave velocity.
The blood pressure developing in a certain blood vessel depends on the pumping output of the heart, on the flow resistance of the blood vessel, on the momentary quantity flowing through, on the elasticity of the blood vessel wall and on the viscosity of the blood.